The use of silica gel as a support for catalysts is well known. The silica gel is a colloidal system of solid character comprised of colloidal particles of a condensation polymerized silicic acid in a hydrated state which forms a coherent structure. It is an assembly of small, impervious, dense, roughly spherical (diameter roughly 100 A) particles in a rather open or loose random packing. The particles are believed to be spherical since the gels are not crystalline. It is believed that the spheres are bonded together by bridges or fillets of the same material. The pore system within the aggregate is formed by the open spaces between the elementary particles and the porous texture, as characterized by the specific surface area, pore volume and pore diameter, depends on the size and the packing of the elementary particles. There are generally two forms of silica gel -- xerogel and aerogel.
An aerogel is a gel in which the liquid phase of a gelled silicic acid solution has been replaced by a gaseous phase in such a way as to avoid the shrinkage which would occur if the gel had been dried directly from a liquid. For example, Kistler prepared silica aerogels by replacing most of the water in the gel with alcohol, heating the gel in an autoclave above the critical temperature of the alcohol so that there was no meniscus between the liquid and gas phases, and venting the vapors. In this way, liquid phase was removed without subjecting the gel structure to the compressive forces due to the surface tension of the liquid-gas interface.
Xerogels are prepared by removal of the water by evaporation from an aqueous gelled silicic acid solution. Evaporation of the liquid phase forms menisci in the pores at the surface of the gel so that the surface tension of the liquid exerts a strong compression on the gel mass. The degree to which the gel can be densified depends on the equilibrium between the compression due to the surface tension and the resistance to compression by the gel framework. Compression will increase with smaller pore diameters; resistance to compression depends upon the strength of the gel which increases with higher packing density and more strongly coalesced structures. Thus, gels of high specific surface, made up of extremely small ultimate silica units and formed at low silica concentration, shrink greatly and crack into fragments upon being dried.
Much of the technology of silica gels involves the problem of making a strong hard gel mass which will not shrink or crack upon being dried and which will be suitable as a catalyst base. On the other hand, there has evolved a considerable art in producing extremely light, friable gels which will break down easily into fine powders for use as fillers in plastics, rubber and the like. This type of xerogel is not suitable for fixed bed catalyst supports.
Other solid forms of silica include the crystalline quartz, tridymite and cristabolite, and these are generally not suitable as catalyst supports because, in part, they are non-porous. The same is true of opal, an amorphous form of silica.
Pelleted diatomaceous earth is a naturally occurring form of siliceous material which is sometimes used as a catalyst support because it has a porous structure and is relatively crush-resistant. However, it also contains alumina and iron impurities which may be harmful to many catalytic reactions.
There is a significant amount of technical literature relating to combining a type of hydrothermal treatment of silica gel with its use as a catalyst support. For example, Czarney et al., Przem. Chem. 46 (4), 203-207 (1967), studied the effect of water pressure (a hydrothermal treatment) and suggested the use of these gels to study the influence of pore structure on catalytic properties. German Offen. 2,127,649 teaches preparing macroporous silica gel spheres by heating them in stream and aqueous ammonia for 3 hours at 10 bars and the resulting material is reported to be useful for catalytic processes. French patent 1,585,305 refers to a method for hardening the surfaces of silica gel without degrading its activity or altering its properties using a heat treatment in a lower alcohol vapor with 10% of its volume as water. Schlaffer et al. J. Phys. Chem. 69 (5), 1530--6 (1965), examined with physical changes that occur to silica and alumina gels upon exposure to steam at moderate to high temperatures and found the surface area and pore volume of silica gel to be less stable to prolonged streaming than those of silica-alumina cracking catalysts.
Other technical literature relates to increasing the crushing strength of silica gel by a steam or water treatment. See, e.g., Bodnikov et al. Zh.Prikl.Khim. 38 (10), 2157-65 (1965) and Sultanov, USSR Patent 281,431. A number of other papers deal with the steam treatment of silica gel to alter pore characteristics.
Micropores are here defined as those measurable by the BET nitrogen adsorption method (see Barrett, The Determination of Pore Volume and Area Distributions In Porous Substances, J. Am. Chem. Soc. 73, 373 (1951)) at P/Po=0.967 which corresponds to pore diameter of 600 A. or less. Macropores are here defined as all other pores contributing to the total porosity. In terms of pore volume, total pore volume, measurable by the method of Innes (Analytical Chemistry 28 No. 3 (March 1956)), comprises pore volume due to micropores (measurable by the BET nitrogen adsorption method) plus pore volume due to macropores. The definition is consistent with one given by Innes.
German Offen. 2,237,015 relates to a phosphoric acid hydration catalyst supported on a treated silica gel carrier. The silica gel carrier material is treated with steam or a mixture of steam and nitrogen at a temperature of 200.degree.-350.degree. C., preferably 250.degree.-300.degree. C., and a pressure of 30-1500 psig to obtain a material of increased crushing strength.
I have found, however, that this treatment irreversibly changes the pore structure of the intermediate density xerogel from one in which most of the pore volume is contributed by the desirable micropores into one in which most of the pore volume is contributed by macropores.
Although the German patent teaches that the steam treatment of the silica gel will increase its strength or wear resistance, it is important to note that the crush strength of the gel is not, per se, transferrable to the catalyst. For example, as is demonstrated in Example 1 below, a sample of virgin grade 57 ID silica xerogel has an average crush strength of 8.6 pounds with 0% equal to or less than 2 pounds. Therefore the phosphoric acid impregnated steam treated silica gel catalysts of the German Offen. could be expected to have a crush strength intermediate between the crush strength of the steam treated gel per se and the same catalyst where the gel has not been steam treated, although not necessarily in excess of the average crush strength of the virgin silica gel.
I have now found that by steam treating silica xerogel by a procedure which is different from the German Offen., a xerogel with a pore structure containing a substantial proportion of desirable micropores, yet also of improved crush strength can be obtained and, very surprisingly, the improved crush strength is transferrable to the support catalyst.
Accordingly, it is the object of this invention to provide an improved phosphoric acid olefin hydration catalyst having substantial microporosity, an average crush strength superior to that obtained in the prior art, and which is prepared from silica gel having a substantial portion of its pore volume contributed by micropores. This and other objects of the invention will become apparent to those skilled in the art from the following detailed description.